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Reconstructing the climate of medieval Europe

Researchers confirm that the medieval warm period was primarily felt in …

One of the uncertainties in climate science is figuring out how global climate trends translate into local predictions and reconstructions, and vice versa. A classic example of this is the medieval warm period, the time when Vikings roamed the North Sea and North Atlantic in shallow-draft open boats. They settled Greenland and made periodic visits to North America to get timber, placing some short-lived settlements there.

But there's a problem. We have the evidence from the adventures of the Norse, but the change to the European climate was so subtle that not many Europeans actually noted it at the time. Apart from exceptional—in both good and bad ways—years, it has been hard to gain a sense of what the typical climate in Europe was like.

To make matters more uncertain, reconstructions of the global climate show little evidence of the medieval warm period—Europe isn't the globe, after all—and some models show a warm period, while others don't. Local indicators, such as pollen grains and tree rings, show mixed evidence for the warm period. A recent paper, published in PLoS One, focuses on calibrating climate reconstructions using European data, but one side effect of the work is a further indication that the medieval warm period was really a local event.

The actual goal of the paper was not to explore the medieval warm period, but to examine Europe's climate over as large period of modern time as possible, and, in doing so, demonstrate a new way of calibrating climate reconstructions.

Historical climate data must be derived from natural thermometers, called proxies: tree ring widths, pollen distributions, ice core data. All of these must be calibrated to get an absolute temperature. Generally, the way to do this is to extrapolate backwards from where the proxy data and instrumental record overlap. In this paper, the researchers used the similarity between tree ring data and pollen distributions as an internal calibration.

The researchers made use of ice core data and glacial records to compare how their local climate reconstructions compared with both a qualitative record—glaciers advance in cold years and retreat in warm years—and a quantitative, but more global, record. They also made use of the instrument record to provide set calibration points for the proxy data.

Understanding the medieval warm period

The end result is a remarkably thorough investigation of the European climate from 750 CE onwards. With the reconstruction complete, the researchers started validating their data.

First, they compared their data with the documented advances and retreats of a glacier. They found good agreement back to 750 CE—in fact the lack of agreement before 750 CE is why they end the reconstruction there—though there were a few years when the glaciers advanced in warm years and retreated in cold years. They also compared their reconstructions to other reconstructions and noted that although there were differences, they were not unreasonably large.

To try and understand the temperature variations, the researchers then examined correlations between their climate reconstruction and known volcanic activity and solar activity. They found at least one cold year in the following three years after major volcanic eruptions. Generally, warm years coincided with years of high solar activity, and vice versa. The one exception was at the end of the medieval warm period; the researchers speculate that deforestation in Europe may have increased the local albedo enough to compensate for the solar activity.

With all that established, what of the medieval warm period? Well, the weather was certainly warmer in Northern Europe and in the North Atlantic, but it was colder and more variable in Southern Europe. After roughly 1400 CE, the less stable weather expanded to Northern Europe and both areas cooled.

The little ice age, on the other hand, was common to most of the continent. The point is that if one used just Northern Europe's tree ring data, the medieval warm period would look very strong, while Southern European proxies would show no warm period. Only by combining data from throughout Europe can one see just how local the medieval warm period actually was.

This is pretty much what climatologists had concluded already—at least as far as the medieval warm period goes. What's important in the new way is that the temperature reconstruction drawn from the proxies resulted in what seems to be a more accurate temperature reconstruction. Because the calibration was internal, the different proxy data sets could be more accurately combined with each other and used to draw much stronger conclusions than previously.

I'm looking forward to seeing this technique applied to other regional climates.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku

88 Reader Comments

Yeahhhh... I don't think I'll be taking ANYTHING in this "report" seriously. The IPCC is one international body that completely destroyed any credibility they had AGES ago.

How so?

nightwing2000 wrote:

OK, let's start by looking at the discredited hockey stick (allegedly) in the NOAA link. Most of the climate info excet the recent "instrumental KRUT" seem basically smooth with variations. The colored lines actually mostly taper off (about 1990?) even below the approx. 1930 peaks. The alarming trend is from two lines - the black "KRUT" line and the thick grey PS2004 line.

What's "KRUT?" The black line is the instrumental temperature record. This is what reflects reality more directly than the proxies because it is a direct measurement of that reality.

Quote:

The PS2004 line seems to have no correlation with reality, if reality is all the other lines. So what is it telling us, other than it's version of reality is much different than anyone else's? It starts well below them all, and smoothly trends upward.

It's a proxie reconstruction of the last 500 years in land climate data from the equator to the arctic circle by Pollack and Smerdon, 2004. The IPCC included a dozen different proxie reconstructions and graphed them together for comparison.

Quote:

The black KRUT line seems to be running with the pack until about 1975 then decides to take off on its own. Again, it bears no relation to reality as depicted by the other lines. SO who's right? Black and grey, or coloured?

Depends on whether you think the direct instrumental record is more accurate, or the proxies.

Quote:

Regardless, perusing records from the last hundreds of thousands of years shows that out current warm spell is a brief interlude, much of the past until about 10K to 20K years ago was much colder.

Oh, you have some new data that our current warm spell will be brief?

Quote:

The last 5 years have NOT been remarkably warm, they've been about average.

Finally, we don't really know WHAT causes ice ages, little ice ages, or all the other climactic variability.

No, we have good ideas for most of them. We also have a good idea what's causing the recent warming trend because accounting for our known impact on the environment is the only way to make the models reflect reality. We have no reason to exclude our own influence from the climate anyway.

Quote:

We see that the sunspots disappeared and there seems to be a correlation between the Mauder min. and cooling; we see that the current sunspot cycle is significantly behind where it should be, including one of the longest sunspot dry spells in recent history...

Which is going on during the warmest decade on record? Do you even think before typing?

Quote:

Basically, I don't know diddly about what is happening but I'm going to pretend my ignorance reflects that of the experts.

It's a "cooked" dataset which fails two of science's most basic tenets: it is neither verifiable nor reproducible. For HadCRUT, the raw measurements, the adjustments and exclusions used, and the detailed rationales for such adjustments and exclusions are not publicly available for other scientists of all stripes to analyze, verify, and reproduce. It is not up to scientific standards, even without the serious questions raised by the inexcusable behaviour and very poor coding exposed by the CRU e-mails and the continuing complaints from other countries and scientists that their data and its accessibility has been misused or miscast by CRU.

It is tainted. It is tainted by substandard scientific practice: it is neither verifiable nor reproducible. It is tainted by the nonscientific ideological biases of CRU leaders as exposed by the CRU e-mails. It is tainted and should be treated as highly suspect. If we accept the proxies used to reconstruct historical climate data as fairly accurate (albeit with varyingly big error bars), good scientific practice would be to compare them to the hadCRUT data and see if they match fairly well or if the comparison casts the hadCRUT line as being substantially skewed. Such a comparison does not make hadCRUT look unbiased. Remove hadCRUT from graphs like those at the top of this page, and suddenly any clear indication of AGW evaporates.

Change timescales from 1000 years to 10,000 years, and suddenly any clear indication of AGW evaporates.

Change timescales again from 10,000 to 100,000 years, and we should be far more concerned about a natural cyclical drop in termperature of up to 8 degrees than we should be about a possible anthropogenic increase of 0.5 to 2 degrees

Evidence for AGW appears to be an artifact of how the data and timescales are being manipulated, not a clear and obvious scientific certainty.

Remove hadCRUT from graphs like those at the top of this page, and suddenly any clear indication of AGW evaporates.

Of course, we can replace the CRUT data with data from NASA or NOAA, and the line of the instrument record is essentially identical. And both of those groups make their code and the data sources available for download.

It's a "cooked" dataset which fails two of science's most basic tenets: it is neither verifiable nor reproducible. For HadCRUT, the raw measurements, the adjustments and exclusions used, and the detailed rationales for such adjustments and exclusions are not publicly available for other scientists of all stripes to analyze, verify, and reproduce.

They aren't? Isn't reproducible in the sense that other, independent instrumental data sets can be checked against it?

Quote:

It is not up to scientific standards, even without the serious questions raised by the inexcusable behaviour and very poor coding exposed by the CRU e-mails and the continuing complaints from other countries and scientists that their data and its accessibility has been misused or miscast by CRU.

Which specific HadCRUT mishandling is covered by the leaked emails? Which countries are saying their data has been misused by the CRU?

Quote:

It is tainted. It is tainted by substandard scientific practice: it is neither verifiable nor reproducible. It is tainted by the nonscientific ideological biases of CRU leaders as exposed by the CRU e-mails.

Which biases are those and how were they exposed? You keep referring to the emails but every official investigation seems to clear them of any scientific misconduct and conclude that the data are reliable. You're arguing from a position that's non-obvious so I'll ask you to support it with proof.

Quote:

It is tainted and should be treated as highly suspect. If we accept the proxies used to reconstruct historical climate data as fairly accurate (albeit with varyingly big error bars), good scientific practice would be to compare them to the hadCRUT data and see if they match fairly well or if the comparison casts the hadCRUT line as being substantially skewed. Such a comparison does not make hadCRUT look unbiased. Remove hadCRUT from graphs like those at the top of this page, and suddenly any clear indication of AGW evaporates.

So replace the HadCRUT data with NASA's and tell us what you find about the proxies vs. instrumental record. It's not like hadCRUT2 is the only instrumental data we have; if you're so convinced that it's fatally flawed (though instead of going that far you only cast aspersions of "taint" and appeal to its allegedly mysterious nature) then use another data set and show us what you come up with.

Quote:

Change timescales from 1000 years to 10,000 years, and suddenly any clear indication of AGW evaporates.

How do you figure? As I've already pointed out, nothing we model can account for the recent warming without the known human influence.

Quote:

Change timescales again from 10,000 to 100,000 years, and we should be far more concerned about a natural cyclical drop in termperature of up to 8 degrees than we should be about a possible anthropogenic increase of 0.5 to 2 degrees

Another interesting graph. Lets see on some things, the angilica ? was discredited for producing bad data. E-mails admit the data was made up. The community can only get published thru the auspices of the UN and that college, Dissenters were dropped out of any discussions and discredited and not given positions in the field to check or to verify. The figures are used worldwide to justify recropping the lands, to not plant trees, to stripmine the countries, and to move jobs from environmentally friendly countries, to no so good places where there are no such controls. Sure NASA used this data, they helped to produce the data, So whats the real picture, can I trust you to tell us the truth? or are you using bad data, from the countries that produced the bad data? Remember GIGO.

Another interesting graph. Lets see on some things, the angilica ? was discredited for producing bad data. E-mails admit the data was made up. The community can only get published thru the auspices of the UN and that college, Dissenters were dropped out of any discussions and discredited and not given positions in the field to check or to verify. The figures are used worldwide to justify recropping the lands, to not plant trees, to stripmine the countries, and to move jobs from environmentally friendly countries, to no so good places where there are no such controls. Sure NASA used this data, they helped to produce the data, So whats the real picture, can I trust you to tell us the truth? or are you using bad data, from the countries that produced the bad data? Remember GIGO.

"We found that our results were accurate back to 750. Cold periods prior to the 20th century can be explained partly by low solar activity and/or high volcanic activity. The Medieval Warm Period (MWP) could be correlated to higher solar activity."

I guess the brighter sun shone only upon western Europe for some reason.

"During the 20th century, however only anthropogenic forcing can explain the exceptionally high temperature rise."

Of course, because only anthropogenic forcing can have global impact, unlike the local effects caused by the sun.

"Warm periods of the Middle Age were spatially more heterogeneous than last decades, and then locally it could have been warmer. However, at the continental scale, the last decades were clearly warmer than any period of the last 1400 years. The heterogeneity of MWP versus the homogeneity of the last decades is likely an argument that different forcings could have operated. These results support the fact that we are living a climate change in Europe never seen in the past 1400 years."

No, the climate change in Europe has been seen in the past 1400 years. The climate change in the rest of the world may or may not be different.

I for one would love to see an explanation of the mechanism that allows a significant portion of the planet to diverge climatically from the rest. Certainly there is no support for such divergence over long time scales in the instrumental record. Over 50 years European climate is strongly correlated with the climate elsewhere on the planet.

The Gulf Stream, the north Atlantic currents, keep northern Europe much warmer than it really should be. Consider Spain is about the same latitude as Michigan/Ohio.... and England about as far north as Quebec... yet their climates are considerably warmer.

This isn't a new thing, archeology has known for many years that in the early first few 100's AD there was a warm spell, because there is evidence of pre-viking civilization in Norway/Sweden successfully farming crops that wouldn't grow well there for another few hundred years... or still, and successful enough to trade with Bronze age cultures, then it all stops for several hundred years before they see Viking settlements. It was the warm spell that let Caesar's Mediterranean armies conquer that far north... and the return to cold that brought down the northern tribes on Rome's head. Archeology has proven the climate to be warmer by the people that lived there's remains, but weather scientists want to know why/how that worked.

I should be a bit more clear. I'd like to see an explanation of the mechanism that allows the climate *trend* for Europe to diverge from that of the rest of the world over long time scales. I am well aware that much of Europe enjoys a boost in temperature due to maritime influences. This should not however affect long term trends, and indeed, when looking at the modern instrumental record, European climate is strongly correlated with global climate. Why was the MWP different?

mabhatter wrote:

joshv wrote:

From the Conclusions:

"We found that our results were accurate back to 750. Cold periods prior to the 20th century can be explained partly by low solar activity and/or high volcanic activity. The Medieval Warm Period (MWP) could be correlated to higher solar activity."

I guess the brighter sun shone only upon western Europe for some reason.

"During the 20th century, however only anthropogenic forcing can explain the exceptionally high temperature rise."

Of course, because only anthropogenic forcing can have global impact, unlike the local effects caused by the sun.

"Warm periods of the Middle Age were spatially more heterogeneous than last decades, and then locally it could have been warmer. However, at the continental scale, the last decades were clearly warmer than any period of the last 1400 years. The heterogeneity of MWP versus the homogeneity of the last decades is likely an argument that different forcings could have operated. These results support the fact that we are living a climate change in Europe never seen in the past 1400 years."

No, the climate change in Europe has been seen in the past 1400 years. The climate change in the rest of the world may or may not be different.

I for one would love to see an explanation of the mechanism that allows a significant portion of the planet to diverge climatically from the rest. Certainly there is no support for such divergence over long time scales in the instrumental record. Over 50 years European climate is strongly correlated with the climate elsewhere on the planet.

The Gulf Stream, the north Atlantic currents, keep northern Europe much warmer than it really should be. Consider Spain is about the same latitude as Michigan/Ohio.... and England about as far north as Quebec... yet their climates are considerably warmer.

This isn't a new thing, archeology has known for many years that in the early first few 100's AD there was a warm spell, because there is evidence of pre-viking civilization in Norway/Sweden successfully farming crops that wouldn't grow well there for another few hundred years... or still, and successful enough to trade with Bronze age cultures, then it all stops for several hundred years before they see Viking settlements. It was the warm spell that let Caesar's Mediterranean armies conquer that far north... and the return to cold that brought down the northern tribes on Rome's head. Archeology has proven the climate to be warmer by the people that lived there's remains, but weather scientists want to know why/how that worked.

I should be a bit more clear. I'd like to see an explanation of the mechanism that allows the climate *trend* for Europe to diverge from that of the rest of the world over long time scales. I am well aware that much of Europe enjoys a boost in temperature due to maritime influences. This should not however affect long term trends, and indeed, when looking at the modern instrumental record, European climate is strongly correlated with global climate. Why was the MWP different?

Basically, there may be a mechanism for solar activity to, in addition to influencing the global climate, alter the distribution of cold and warm air in a way that influences Northern Europe significantly. So, that could explain why a small change in solar activity could produce a larger climate difference in a subset of Europe. As for the difference with the present day, the cause is different - greenhouse forcings vs. solar - so you wouldn't necessarily expect an identical pattern.